This invention relates generally to image forming optical systems, and more particularly the invention relates to phase shifting test mask patterns for characterizing illumination and mark quality in such systems.
A phase shifting mask alters the phase of light passing through certain areas of a photo mask in order to utilize destructive interference to improve resolution and depth of focus in optical lithography. The semiconductor industry is now considering such masks for use in fabricating integrated circuits such as mega bit memory and microprocessors which contain millions of transistors.
The phase shifting mask and its use in an image forming optical system must be free of printable defects. To this end, a mask with phase shifted regions can act as a precision instrument for characterizing the imaging.